Opthalmic lens preform

ABSTRACT

The invention concerns an ophthalmic lens preform having intrinsically at least a mechanical angular indexing member ( 7, 8 ). It is applicable to the manufacture of lenses in mineral or organic material.

The present invention relates, generally, to the production ofophthalmic lenses more commonly known as spectacle lenses. In relatesmore specifically to a lens preform in the form of an unfinished blankarising from molding—in a synthetic or glass material—and from which afinished lens producing the prescribed optical correction is obtained byremoval of material, i.e. surfacing operations.

The ophthalmic lens production process generally comprises two mainsteps.

First, a blank or preform is obtained by molding the synthetic or glassmaterial selected to constitute the basic substrate of the lens. Eachmold employed for this operation has a shape and dimensions designed tocover a certain range of prescriptions and can even, for the most commonprescriptions, have been dedicated to the large scale production offinished lenses having surfaces directly shaped to the desiredprescriptions. But for less common prescriptions and/or more generallywith an aim to simplification and logistics and, as a corollary to this,to reducing the number of molding lines, it may be preferable torestrict molding lines to the large scale production of a reduced numberof, or even one single, standard preforms. Such preforms, which are asort of semi-finished lens blank, are then the object of a certainnumber of finishing operations to arrive at the finished lens having theoptical properties specifically required.

Thus, in the second major operation, the blank from the moldingoperation is surfaced on one and/or the other face to correspond to thegeometric design and prescribed correction. Because of the tightprecision and quality of surface requirements that apply, this surfacingoperation is split up into several sub-steps each involving a specificworkstation, thereby requiring the lens to circulate during surfacingfrom one station to the other. We thus generally distinguished, forsurfacing each face of the lens, one grinding station providing bothrough cutting and finishing with two separate tools, and a polishingstation optionally preceded by a buffing station. Between the stations,or following them, other workstations for the preform/lens (for exampleengraving, checking, etc stations) may be present. Work on thepreform/lens on each of these supplementary stations, with accurate andcontrolled positioning, may prove to be necessary.

One of the problems most frequently encountered during this surfacingprocess and, more generally, work on the semi-finished lens, resides inthe successive positioning and repositioning of the blank at each of itstransfers from one station to the other. This recurring sub-operation ofpositioning and repositioning the workpiece, known more commonly as“blocking” the lens, is particularly delicate, expensive, and canfrequently lead to inaccuracies of positioning. It will indeed beunderstood that the lens preform, of transparent synthetic or glassmaterial, is relatively fragile and must be preserved from any marking,especially on that one of its two faces which is finished while work isproceeding on its other face. Further, and above all, the lens requiresto be positioned highly accurately at each station, with a spatialorientation that is known in a reference frame determined for thestation concerned. In particular, when handling progressive lenses inwhich symmetry of revolution is absent, the angular position of theblank about its central axis must be mastered accurately.

Several ways of “blocking” a semi-finished lens for working on it, andin particular surfacing it, are known. The most widely used andpractical method at the present time consists in forming and securing bymelting, on one of the faces of the lens, a securing metal block madefrom a low melting point alloy and having clasping means adapted toco-operate with jaw- or mandrel-type gripping members on the variousworkstations participating. This method is, overall, satisfactory, butsuffers from several disadvantages.

It is, firstly, expensive and relatively tricky to implement. The lowmelting point alloys employed are in effect relatively expensive andshould be considered as pollutants dangerous for the environment so muchso that it is necessary, both for economic reasons as well as compliancewith stricter environmental constraints, to arrange for carefulrecycling. But, even with effective recycling, loss of alloy byevaporation during melting cannot be avoided.

Further, apart from these economic and environmental aspects, each lensbeing surfaced needs to be transported from one station to the otherwith its holding block, which is a hindrance not only during transfer,but also on the occasion of possible storage or transport between twooperations. There is a minimum time before the lens associated with itsholding blocks required on a machining station (around 15 minutes), aswell as a maximum time beyond which machining cannot be implemented(around 24 hours), these periods of time consequently settingconstraints on the workflow for such lenses. Further, in the case ofstorage or prolonged waiting between two operations, it would beexcessively costly to provide a number of holding blocks in progressequivalent to the number of lenses waiting. It thus becomes necessarysometimes, between two operations, to release the lens from its initialholding block in order to transfer, store or transport it more readily.It is then necessary, when the process starts again, to again associateit with a fresh holding block, with the practical difficulties arisingtherefrom, not only as regards softening the low melting point alloy butalso as regards mastering geometrical aspects of further work on thelens blank, and the associated cost. It is indeed necessary, notably, asmentioned above, in the case of complex-surface lenses without symmetryof revolution, to know the relative geometry of the lens with respect toeach workstation concerned and, at the same time, with respect to theholding block associated with the lens blank. It is consequentlynecessary to perform, in some manner, a reading of the face of the lenscurrently being finished. Such a reading constitutes, each time work isrestarted on the lens, a supplementary complex, and consequentlyinaccurate and costly, operation.

Vacuum systems have also been proposed for blocking lens preforms. Suchsystems implement grasping means which, in order to constitute a type ofsuction cup, have a cavity surrounded by an annular seal against whichthe preform abuts in order to define together with the cavity and itsseal, a chamber where a relative vacuum is set up via an associatedvacuum pump. Although this does not have the same economic andenvironmental disadvantages as the molten block solution discussedabove, this type of blocking is little used in practice, and has proveddifficult to implement for complex (i.e. non spherical) surfaces withwhich the seal did not provide a sufficiently accurate abutment.Additionally, angular alignment, when this is provided, is done bybringing registration marks provided respectively on the preform and thesuction cup into visual registration. This purely visual registration isa source of inaccuracy and/or complication when blocking thepreform/lens.

The general aim of this invention is to facilitate these operations ofpositioning and repositioning semi-finished lens blanks or preforms,with constant geometry.

There is consequently proposed, according to the invention, anophthalmic lens preform or blank which has, intrinsically, at least oneelement for mechanical angular registration.

It is thus possible, at each step in the work process, to employ thisintrinsic registration element on the preform in combination with amating element on the workstation concerned, to ensure positioning orrepositioning of the preform with an accurate and known spatialorientation and in particular, angular registration in the workstationreference frame.

According to an advantageous characteristic, the preform has twodiametrically opposed members for mechanical registration which areseparated from each other by at least one of their dimensions. Thisincreases the accuracy and reliability of alignment. The fact of the tworegistration elements being different avoids any possibility of a 180°or so angular positioning error of the preform.

In one embodiment offering considerable scope and great flexibility inuse, the element for angular registration takes the form of a notchprovided in the actual body of the preform. By the term “notch” we meanany mark whatever its form may be, taking the form of a hollow and not aprojection. The registration element is consequently of a “subtractive”nature, which in particular allows it to be provided either directly byinjection of the preform, or in a preparatory machining phase formaterials which are not suitable for injection. Further, the absence ofan external protuberance on the preform is of itself advantageous assuch a protuberance could hinder manipulation and storage of thepreform, as well as increase workstation cycle time (grinding inparticular).

For a preform having two diametrically opposed registration notches, thetwo notches are preferably distinguished one from the other by theirdepth.

According to another advantageous characteristic of the invention, thepreform has two main faces one of which is concave, a peripheral edgejoining these two faces. Preferably, the notch, or at least one of thenotches if several are provided, is arranged on the concave face, at theouter periphery thereof, which favors a more accurate axial positioningin view of its diameter.

For ease of manufacture and for greater efficiency of alignment, thenotch is open inwardly at the concave face and outwardly on the edge ofthe preform. In a preferred embodiment, the notch has a V-shapedcross-section profile the direction of which is perpendicular to thecentral axis of the preform. This widened-out shape of the notch givesit self-centering capabilities (for angular self-centering of thepreform), which facilitates positioning or repositioning of the preform,notably when it is being placed in position by an automatic loadingunit. It may be desirable both in manufacture and in use, for the baseof the notch to be smoothed off. For example, the bottom of the notchcan have a circular cross section.

More precisely, the circular cross-section of the base of the notch hasa radius of about 1.5 mm. The V-section arms of the notch make an angleof about 45° with each other.

According to a further advantageous characteristic of the invention, theedge of the preform is, overall, undercut (substantially conical) overits major portion, to allow it to be obtained by an injection/moldingprocess, but has a substantially cylindrical clamping portion adjacentto the concave face, designed to allow the preform to be gripped andclamped at the various workstations concerned.

Typically, the thickness of the clamping portion of the preform edge issubstantially less than one third of the total thickness of the edge.However, the thickness of the clamping portion of the edge willpreferably remain greater than the depth of the notch, which opens outat this point as well as over its whole depth. The thickness ispreferably greater than or equal to 5 mm.

According to a further advantageous characteristic of the invention, theconcave face is bordered by a flat annular abutment rim allowing stableand accurate seating of the preform against a corresponding supportsurface of the workstation concerned. When the edge of the preform isprovided with a cylindrical claming portion, the plane of the annularabutment rim is, in a preferred embodiment, perpendicular to the axis ofthis cylindrical clamping portion.

Further characteristics and advantages of the invention will become moreclear from the description which follows of one particular embodiment,given by way of non limiting example.

Reference is made to the attached drawings in which:

FIG. 1 shows an ophthalmic lens preform according to the invention inprofile.

FIG. 2 shows detail of the region II in FIG. 1.

FIG. 3 shows a section along line III—III of FIG. 1.

FIG. 4 is a plan view according to arrow IV in FIG. 1.

With reference to the drawings, an ophthalmic lens preform according tothe invention has two principal faces, a concave face 1 and a convexface 2, and a peripheral edge 3 joining these two faces.

Conventionally, the edge of the preform is slightly conical, andconsequently undercut, over the major portion 4 of its thickness, toallow injection/molding. Edge 3 has, over a minor proportion of itsthickness, a substantially cylindrical clamping portion 5, adjacent toconcave face 1.

Typically, the thickness e, of clamping portion 5 of edge 3 issubstantially less than one-third of the total thickness, E, of saidedge. Still, the thickness e of clamping portion 5 of edge 3 remainsgreater than the depth p1, p2 of each of the two notches 7, 8, whichopen out at this point as well as over their whole depth. In practice,the thickness e is preferably greater than or equal to 5 mm and in theexample illustrated, is around 5 mm.

Concave face 1 is bordered by a flat annular abutment rim 6, allowingstable and accurate seating of the preform against a correspondingsupport surface on the workstation concerned. The plane of this annularabutment rim 6 is perpendicular to the axis of this clamping portion 5.

The preform further has two notches 7, 8 which, formed in the actualmaterial of the preform, constitute intrinsic elements (i.e. integral,originating from injection/molding, or obtained previously by othermeans depending on the nature of the materials employed) for mechanicalregistration on a mating element on each one of the various workstationsat which the preform needs to be positioned or repositioned. A notchhere is any mark, whatever its shape may be, constituting a hollow andnot a relief, of sufficient dimensions to mechanically co-operate with amatching registration element on the workstation concerned. Suchmechanical co-operation should be clearly distinguished from opticalco-operation. To get a clear idea, one should thus consider that thedimensions (length, width or diameter, and depth) of each notch are allgreater than 1 mm.

Registration notches 7, 8 are diametrically opposed and aredistinguished from each other by at least one of their dimensions, inthe case considered here, their depth. Because of this arrangement onthe outer diameter of the preform, accuracy and reliability of angularalignment are improved. The fact that the two registration elements aredifferent avoids any possible angular positioning error of around 180°.

The registration notches 7, 8 are, in the case considered, provided onconcave face 1, at the periphery thereof. For ease of manufacture andfor greater alignment effectiveness, each notch opens inwardly at theconcave face and externally at the edge of the preform.

More precisely, in the embodiments illustrated, each registration notch7, 8 has a V-section in profile the direction of which is substantiallyperpendicular to the central axis of the preform. This widened-out formof the notch gives it a self-centering capability facilitatingpositioning or repositioning of the preform, with automatic angularalignment of the preform.

To facilitate manufacture and use, the base of the notch is rounded offand has, more precisely, a 1.5 mm radius circular section. The two armsof the V-section of each notch 7, 8 make an angle of 45° with eachother.

Tests allowed us to determine the following advantageous geometricalcharacteristics, corresponding to those of the embodiment illustrated:

-   -   concave face 1, which is substantially spherical, has a radius        r1 comprised between infinity (face 1 is then flat) and 50 mm,        for example, as is the case here, about 71 mm;    -   convex face 2, which is substantially spherical, has a radius r2        comprised between 60 mm and infinity (face 2 is then flat), for        example, as is case here about 301 mm and;    -   each registration notch 7,8 has a depth, referenced respectively        p1, p2, comprised between 3 and 6 mm, and the difference in        depth p2−p1 between the two notches 7 and 8 is at least 1 mm;        for example, as is the case here, the two notches 7,8 have        respective depths p1 and p2 of 3.4 and 4.4 mm, with the        difference in depth (p2−p1) between the two notches being 1 mm;    -   the total thickness e of the edge 3 of the preform is comprised        between 12 and 25 mm, for example, as is the case here, about        23.5 mm;    -   the diameter of the preform can vary between 60 and 100 mm        depending on the diameter of the finished lenses employed (in        particular, but not restrictively for reasons of material        removal optimization), for example, as is the case here, about        81 mm.

The invention is not limited to the particular embodiments justdescribed but covers, on the contrary, any alternative embodimentrepresenting, with equivalent means, the essential characteristicsthereof. In particular, although a preform or blank having twodiametrically opposed registration notches which differ in depth fromeach other have been described and illustrated, it would be equallypossible, without departing from the scope of the invention, to providea preform or blank with two diametrically opposed notches which differedfrom each other in some other dimension (for example, their width orlength) or, yet again, an odd number of notches (typically, one or threenotches) automatically providing a definite registration over 360°,without possible error of around 180°.

1. An ophthalmic lens preform or blank, having, intrinsically, at leastone element for mechanical angular registration, wherein the perform hastwo main faces, one of which is concave, and a peripheral edge joiningthe said two faces, wherein the edge of the preform has a major portionand a clamping portion, with the major portion being undercut and theclamping portion being substantially cylindrical adjacent to the concaveface.
 2. The preform of claim 1, having two diametrically opposedelements for mechanical registration, said elements differing from eachother by at least one of their dimensions.
 3. The preform of claim 1,wherein said element is a notch formed in a body of said preform.
 4. Thepreform of claim 2, wherein said elements are notches and differ fromeach other by their respective depths.
 5. The preform of claim 1,wherein said element is a notch provided on the concave face, at theperiphery thereof.
 6. The preform of claim 5, wherein the notch opensinwardly at the concave face and outwardly at the edge of the preform.7. The preform of claim 5, wherein the notch has a V-section profile thedirection of which is substantially perpendicular to a central axis ofthe preform.
 8. The preform of claim 7, wherein a base of the notch issmoothed off.
 9. The preform of claim 8, wherein a base of the notch hasa circular section.
 10. The preform of claim 9, wherein the circularcross-section of the base of the notch has a radius of about 1.5 mm. 11.The preform of claim 7, wherein arms of the V-section of the notch makean angle with each other of about 45°.
 12. The preform of claim 5,wherein the notch has a depth comprised between 3 and 6 mm.
 13. Thepreform of claim 4, wherein each of said notches has a depth comprisedbetween 3 and 6 mm and wherein the difference in depth between the twonotches is at least 1 mm.
 14. The preform of claim 1, wherein athickness of the clamping portion of the edge is substantially less thanone third of the total thickness of said edge.
 15. The preform of claim14, wherein the notch has a V-section profile the direction of which issubstantially perpendicular to a central axis of the preform and whereinthe thickness of the clamping portion of the edge is greater than thedepth of the notch.
 16. The preform of claim 14, wherein the thicknessof the clamping portion of the edge is greater than or equal to 5 mm.17. The preform of claim 16, wherein the thickness of the clampingportion of the edge is about 5 mm.
 18. The preform of claim 1, whereinthe periphery of the concave face is bordered by a flat annular abutmentrim.
 19. The preform of claim 18, wherein the flat annular abutment rimis perpendicular to the cylindrical clamping portion of the preformedge.
 20. The preform of claim 1, wherein the other face of which isconvex.
 21. The preform of claim 20, wherein: the concave face issubstantially spherical and has a radius comprised between infinity and50 mm; the convex face is substantially spherical and has a radiuscomprised between 60 mm and infinity; the total thickness of the edge ofthe preform is comprised between 12 and 25 mm.
 22. The preform of claim21, wherein: the concave face has a radius of about 71 mm; the convexface has a radius of about 301 mm; the total thickness of the edge ofthe preform is about 23.5 mm wherein the perform has two diametricallyopposed elements for mechanical registration, each element being anotch, wherein the two notches have respective depths of 3.4 and 4.4 mm,whereby the difference in depth between the two notches is 1 mm.
 23. Thepreform of claim 1, the diameter of which is comprised between 60 and100 mm.
 24. The preform of claim 2, wherein said element is a notchformed in a body of said preform.